This invention relates to processes for coagulating polymer latices, and more particularly to processes for coagulating such latices using a screw-type extruder.
As used herein, the term "polymer latex" means an emulsion of a polymer in an aqueous medium. The polymer may be either a conventional synthetic rubber polymer such as a styrene-butadiene rubber (SBR), a polychloroprene rubber (neoprene), an acrylonitrile-butadiene rubber (nitrile rubber), etc.; a conventional naturally occurring rubber latex such as a hevea rubber, a balata rubber, a gutta percha rubber, a guayule rubber, etc.; a conventional plastic polymer such as an acrylonitrile-butadiene-styrene resin (ABS resin), etc.; or a mixture or any other combination thereof such as an ABS-extended SBR; and as used herein, the term "polymer" means any of the foregoing materials. The term "rubber latex" as used herein means a latex of a conventional rubber polymer such as an SBR, neoprene, nitrile, hevea, balata, gutta percha, guayule, etc., which is used to make rubber. The "solids content" of a polymer latex refers to the weight percent of polymer and other solids in the latex.
A polymer latex is generally the product of an emulsion polymerization of a polymerizable material, although in the case of naturally occurring rubber latices, the latex is the product of a primarily natural process. The polymer is recovered from the polymer latex by coagulating the latex. This is accomplished by mixing the polymer latex with a coagulant which breaks down the emulsion of the polymer. Any of a wide variety of conventional coagulants can be used for this purpose. Known coagulants include electrolytes such as acids, salts, and aqueous solutions of acids and salts. Suitable aqueous acids are aqueous H.sub.2 SO.sub.4, HCl, H.sub.3 PO.sub.4, HC.sub.2 H.sub.3 O.sub.2, and the like. Aqueous H.sub.2 SO.sub.4 is particularly preferred for coagulating rubber latices, and particularly SBR. Suitable aqueous salt solutions are solutions of Ca(NO.sub.3).sub.2, Al.sub.2 (SO.sub.4).sub.3, NaCl, and the like. Other known coagulants are organic solvents such as ketones (e.g., acetone), alcohols (e.g., ethyl alcohol, methyl alcohol, etc.), and mixtures thereof. Mixtures of electrolytes and organic solvents (e.g., an emulsion of an alcohol in an electrolyte) may also be used as coagulants.
In the coagulation of polymer latices, complete coagulation of the latex is the desired objective. The amount of coagulant required to produce complete coagulation may be an important consideration in the economics of the coagulation process. Polymer latices which are easy to coagulate are frequently difficult to completely coagulate. This is because the rapidly coagulating polymer encapsulates or otherwise traps uncoagulated latex and prevents coagulant from contacting the encapsulated latex as is required to complete coagulation. Neoprene is an example of a polymer which is particularly troublesome in this respect.
It is also desirable to produce a coagulated polymer with low moisture content since this facilitates handling and further processing of the polymer. Many known coagulation processes produce coagulated polymer with relatively high moisture content, e.g., 40-50% moisture by weight. The excess liquid of the coagulation process (i.e., the liquid which separates from the coagulated polymer) should also be relatively low in polymer since it is typically uneconomical to attempt to recover any polymer from this liquid and this polymer is therefore usually lost. In other words, the coagulation process preferably gives good separation between the coagulated polymer and the excess process liquid.
It may also be desirable to produce coagulated polymer which has been compacted and agglomerated since this promotes separation of the coagulated polymer from the excess process liquid and facilitates further handling of the polymer.
In view of the foregoing, it is an object of this invention to provide improved processes for coagulating polymer latices.
It is a more particular object of this invention to provide processes for coagulating polymer latices which assure substantially complete coagulation of the polymer in the latex.
It is another more particular object of this invention to provide processes for coagulating polymer latices which reduce the amount of coagulant required for complete coagulation of the polymer in the latex.
It is still another more particular object of this invention to provide processes for coagulating polymer latices which give good separation between the coagulated polymer and the excess process liquid, which produce compacted coagulated polymer having relatively low moisture content, and excess process liquid which is relatively low in uncoagulated polymer.